Various types of weighing apparatus (e.g., scales) are known and would be familiar to one of skill in the art. These apparatus may include static weighing scales such as, without limitation, electronic analytic and laboratory scales, retail scales, and industrial scales.
An apparatus in the form of a checkweigher for weighing moving objects (e.g., packages) is also known, and various checkweigher designs exist for this purpose. In general, however, a checkweigher may be described as a high-speed weighing device for the in-motion weighing of objects as the objects travel along a conveyor. A checkweigher is typically installed to a conveyor line such that an infeed conveyor is provided for delivering objects to the checkweigher and a discharge conveyor is provided to transport objects from the checkweigher to a downstream location. The checkweigher itself also typically employs a conveyor to transport objects one at a time across a static scale, which is essentially a vertically deflectable mechanism operable to effect weighing of the moving objects.
All weighing scales, including checkweighers, employ a sensor that transforms the weight of an object into a usable signal. Typically, this signal is converted into a readable value by some type of analog-to-digital (A/D) converter. Sensors typically used for this purpose include, for example, strain gauge load cells, electromagnetic force restoration weigh cells, or other sensors such as capacitive or inductive sensors.
Regardless of the specific type, all such checkweighers or other weighing apparatus will, over time, typically experience a slight change in weighing characteristics. Such a change in weighing characteristics may be due to, for example, a buildup of material on the load receiving surface (e.g., weighpan or load platform) and/or to the drift (change in output under a constant load) of the weighing sensor (e.g., load cell) as a result of changes in operating temperature, etc. This is particularly true for weighing apparatus that use sensors that are not highly accurate, such as strain gauge load cells. To a lesser degree, this is also true for weighing apparatus that use more accurate sensors, such as electromagnetic force restoration load cells.
As a consequence of the aforementioned slight change in weighing characteristics, the checkweigher or other weighing apparatus is typically required to go through a rezero operation on some periodic basis or in response to the observation of certain production parameters. A rezero operation reestablishes the zero value of the weighing apparatus. That is, the rezero operation allows the weighing apparatus to compensate for the environmental or other factors that have changed its weighing characteristics and to once again read zero when in an actual unloaded state.
A rezero operation is accomplished by causing the weighing apparatus to take a deadload weight measurement. Normally, the deadload weight of a weighing apparatus is the weight of the unloaded weighing apparatus weighpan or load platform. Therefore, during a rezero operation, the weighing apparatus is required to take a weight reading when no object is present on the weighpan or load platform and, if the deadload weight reading differs from a previous deadload weight reading, to establish a new zero value at the current deadload weight reading value (i.e., to rezero the weighing apparatus).
In the case of checkweighers, a periodic gap in production (i.e., a gap between objects crossing the scale thereof) is required in which to reestablish the zero value (to perform a rezero operation). Given a sufficient gap in production, a checkweigher may be programmed to automatically measure the deadload weight of the scale weighpan, which deadload weight measurement will become the new zero value provided it does not exceed a specified threshold (e.g., 2% of the scale maximum scale load). Violations of the rezero threshold may occur due to the conditions noted earlier: product buildup or drift. If a good rezero point is not established on a regular basis, the accuracy of the checkweigher will likely be reduced.
To ensure proper weighing accuracy, a checkweigher or other weighing apparatus may be designed to require a rezero on a periodic basis. From a last good rezero, a checkweigher or other weighing apparatus may be required to rezero again on some predetermined schedule, such as by using either a timer or an item counter. When such a rezero point occurs, a checkweigher may attempt to rezero as soon as a sufficient gap in production is presented. Upon a successful rezero, the timer or other trigger associated with the checkweigher or other weighing apparatus is usually reset. However, if a new rezero cannot be established within some allotted time frame, then the checkweigher or other weighing apparatus typically enters a “needs rezero” condition, which is also commonly referred to as a rezero fault.
Known checkweighers and other weighing apparatus may alert an operator of a rezero fault, such as by energizing a light or buzzer, illuminating a section of the human-machine interface (HMI) display, etc. In the case of a checkweigher, an operator can typically remedy a rezero fault by either manually providing a gap in production (i.e., removing items from conveyor to produce a gap) that is sufficient to allow a rezero, or by cleaning the weighpan to remove product buildup.
If a rezero cannot be completed by the weighing apparatus, then use of the weighing apparatus may need to be suspended because continued use may result in inaccurate weight readings. For example, in the case of a checkweigher, the infeed conveyor may be stopped so that no additional product is transferred over the scale and weighed, or the checkweigher may be shut down entirely. This is, of course, problematic at least in the sense that it interrupts production and will remain an interruption until an operator or other personnel can remedy the rezero fault. In a situation where the checkweigher or other weighing apparatus operates unattended or where only limited operator support is available, such a situation can seriously impact production.
Ultimately, checkweighers and other weighing apparatus must be periodically rezeroed in order to ensure the proper and accurate operation of the weighing function. It is obviously desirable that rezeroing occurs as required with as little as possible risk of process failure or process interruption. Consequently, it can be understood that a rezero monitoring system and method that tracks each rezero operation and identifies any trend toward an inevitable rezero fault would be beneficial to preventing a future interruption of weighing apparatus operation. Further, the drawbacks associated with requiring an operator to be constantly or very frequently present at a weighing apparatus should also be apparent, and eliminating such a requirement would be desirable as well. It can, therefore, be understood that a rezero monitoring system and method that notifies operators and/or other remotely located interested parties of a predicted or actual rezero fault would also be beneficial. System and method embodiments according to the invention provide such benefits.